Rollup merge of #134920 - lqd:polonius-next-episode-6, r=jackh726

Convert typeck constraints in location-sensitive polonius

In this PR, we do a big chunk of the work of localizing regular outlives constraints.

The slightly annoying thing is handling effectful statements: usually the subset graph propagates loans at a single point between regions, and liveness propagates loans between points within a single region, but some statements have effects applied on exit.

This was also a problem before, in datalog polonius terms and Niko's solution at the time, this is about: the mid-point. The idea was to duplicate all MIR locations into two physical points, and orchestrate the effects with that. Somewhat easier to do, but double the CFG.

We've always believed we didn't _need_ midpoints in principle, as we can represent changes on exit as on happening entry to the successor, but there's some difficulty in tracking the position information at sufficient granularity through outlives relation (especially since we also have bidirectional edges and time-traveling now).

Now, that is surely what we should be doing in the future. In the mean time, I infer this from the kind of statement/terminator where an outlives constraint arose. It's not particularly complicated but some explanation will help clarify the code.

Assignments (in their various forms) are the quintessential example of these crossover cases: loans that would flow into the LHS would not be visible on entry to the point but on exit -- so we'll localize these edges to the successor. Let's look at a real-world example, involving invariance for bidirectional edges:

```rust
let mut _1: HashMap<i32, &'7 i32>;
let mut _3: &'9 mut HashMap<i32, &'10 i32>;
...
/* at bb1[3]: */ _3 = &'3 mut _1;
```

Here, typeck expectedly produces 3 outlives constraints today:
1. `'3 -> '9`
2. `'7 -> '10`
3. `'10 -> '7`

And we localize them like so,

1. `'3 -> '9` flows into the LHS and becomes: `3_bb1_3 -> 9_bb1_4`
2. `'7 -> '10` flows into the LHS and becomes: `7_bb1_3 -> 10_bb1_4`
3. `'10 -> '7` flows from the LHS and becomes: `10_bb1_4 -> 7_bb1_3` (time traveling 👌)

---

r? ``@jackh726``

To keep you entertained during the holidays I also threw in a couple of small changes removing cruft in the borrow checker.

We're actually getting there. The next PR will be the last one needed to get end-to-end tests working.

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -10,6 +10,7 @@ use rustc_hir::intravisit::Visitor;
 use rustc_hir::{self as hir, BindingMode, ByRef, Node};
 use rustc_middle::bug;
 use rustc_middle::hir::place::PlaceBase;
+use rustc_middle::mir::visit::PlaceContext;
 use rustc_middle::mir::{
     self, BindingForm, Local, LocalDecl, LocalInfo, LocalKind, Location, Mutability, Place,
     PlaceRef, ProjectionElem,
@@ -22,7 +23,6 @@ use rustc_trait_selection::traits;
 use tracing::debug;
 
 use crate::diagnostics::BorrowedContentSource;
-use crate::util::FindAssignments;
 use crate::{MirBorrowckCtxt, session_diagnostics};
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
@@ -1088,6 +1088,38 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         }
     }
 
+    /// Finds all statements that assign directly to local (i.e., X = ...) and returns their
+    /// locations.
+    fn find_assignments(&self, local: Local) -> Vec<Location> {
+        use rustc_middle::mir::visit::Visitor;
+
+        struct FindLocalAssignmentVisitor {
+            needle: Local,
+            locations: Vec<Location>,
+        }
+
+        impl<'tcx> Visitor<'tcx> for FindLocalAssignmentVisitor {
+            fn visit_local(
+                &mut self,
+                local: Local,
+                place_context: PlaceContext,
+                location: Location,
+            ) {
+                if self.needle != local {
+                    return;
+                }
+
+                if place_context.is_place_assignment() {
+                    self.locations.push(location);
+                }
+            }
+        }
+
+        let mut visitor = FindLocalAssignmentVisitor { needle: local, locations: vec![] };
+        visitor.visit_body(self.body);
+        visitor.locations
+    }
+
     fn suggest_make_local_mut(&self, err: &mut Diag<'_>, local: Local, name: Symbol) {
         let local_decl = &self.body.local_decls[local];
 
@@ -1121,7 +1153,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             })) => {
                 // check if the RHS is from desugaring
                 let opt_assignment_rhs_span =
-                    self.body.find_assignments(local).first().map(|&location| {
+                    self.find_assignments(local).first().map(|&location| {
                         if let Some(mir::Statement {
                             source_info: _,
                             kind: